Semiconductor devices may be roughly classified as lateral semiconductor devices wherein electrodes are arranged on a major surface and vertical semiconductor devices wherein electrodes are distributed on both major surfaces opposing each other. When the vertical semiconductor device is ON, a drift current flows in the expansion direction of a drift layer, which becomes depleted by the reverse bias voltage when the vertical semiconductor device is OFF. FIG. 19 is a cross section of a conventional planar n-channel vertical MOSFET. Referring now to FIG. 19, this vertical MOSFET includes a drain electrode 18; an n+-type drain layer 11 with low resistance, to which drain electrode 18 is in electrical contact; a highly resistive n−-type drift layer 12 on n+-type drain layer 11; a p-type base region 13a selectively formed in the surface portion of n−-type drift layer 12; a heavily doped n+-type source region 14 selectively formed in p-type base region 13a; a gate electrode layer 16 above the extended portion of p-type base region 13a extended between n+-type source region 14 and n−-type drift layer 12; a gate oxide film 15 between gate electrode layer 16 and the extended portion of p-type base region 13a; a source electrode 17 in common contact with the surfaces of n+-type source region 14 and p-type base region 13a; and a drain electrode 18 on the back surface of n+-type drain layer 11.
In the vertical semiconductor device as shown in FIG. 19, highly resistive n−-type drift layer 12 works as a region for making a drift current flow vertically when the MOSFET is in the ON-state. Highly resistive n−-type drift layer 12 is depleted when the MOSFET is in the OFF-state, resulting in a high breakdown voltage of the MOSFET. Shortening the current path in highly resistive n−-type drift layer 12 is effective for substantially reducing the on-resistance (resistance between the drain and the source) of the MOSFET, since the drift resistance is lowered. However, the short current path in n−-type drift layer 12 causes breakdown at a low voltage, since the expansion width of the depletion layer that expands from the pn-junction between p-type base region 13a and n−-type drift layer 12 is narrowed and the electric field strength soon reaches the maximum (critical) value for silicon. In a semiconductor device with a high breakdown voltage, the characteristically thick n−-type drift layer 12 causes high on-resistance and therefore, losses increase. In short, there exists a tradeoff between the on-resistance and the breakdown voltage of the MOSFET. This tradeoff between the on-resistance and the breakdown voltage also exists in other semiconductor devices such as IGBT's, bipolar transistors and diodes. The tradeoff between the on-resistance and the breakdown voltage is also present in lateral semiconductor devices, in which the flow direction of the drift current in the ON-state of the devices is different from the expansion direction of the depletion layer in the OFF-state of the device.
EP0053854, U.S. Pat. Nos. 5,216,275, U.S. Pat. No. 5,438,215 and Japanese Unexamined Laid Open Patent Application H09 (1997)-266311 disclose semiconductor devices that include adrift layer including heavily doped n-type regions and p-type regions alternately laminated with each other to solve the foregoing problems. The alternately laminated n-type regions and p-type regions are depleted to bear the breakdown voltage in the OFF-state of the device.
FIG. 20 is a cross section of a part of the vertical MOSFET according to an embodiment of U.S. Pat. No. 5,216,275. The vertical MOSFET of FIG. 20 is different from the vertical MOSFET of FIG. 19 in that the vertical MOSFET of FIG. 20 includes a drift layer 22, that is not single-layered, but consists of n-type drift regions 22a and p-type drift regions 22b alternately laminated with each other. In the figure, there is a p-type base region 23a, an n+-type source region 24, a gate electrode 26, a source electrode 27, and a drain electrode 28.
Drift layer 22 is formed in the following manner. First, a highly resistive n-type layer is grown epitaxially on an n+-type drain layer 21. The n−-type drift regions 22a are formed by etching the highly resistive n-type layer to form trenches down to n+-type drain layer 21. Then, p-type drift regions 22b are formed by epitaxially growing p-type layers in the trenches.
Hereinafter, the semiconductor device, including an alternating conductivity type drift layer that makes a current flow in the ON-state of the device and is depleted in the OFF-state of the device, will be referred to as a “semiconductor device with an alternating conductivity type layer.”
The dimensions described in U.S. Pat. No. 5,216,275 are as follows. When the breakdown voltage is put in VB, the thickness of the drift layer 22 is 0.024VB1.2 (μm). When n-type drift region 22a and p-type drift region 22b have the same thickness b and the same impurity concentration, the impurity concentration is 7.2×1016 VB−0.2/b (cm−3). If VB is 800 V and b μm, the drift layer 22 will be 73 μm in thickness and the impurity concentration 1.9×1016 cm−3. Since the impurity concentration for the single-layered drift layer is around 2×1014 cm−3, the on-resistance is reduced. However, when using conventional epitaxial growth techniques, it is difficult to bury a good quality semiconductor layer in such a narrow and deep trench (with a large aspect ratio).
The tradeoff between the on-resistance and the breakdown voltage is also commonly encountered in lateral semiconductive devices. The foregoing references, EP0053854, U.S. Pat. No. 5,438,215 and Japanese Unexamined Laid Open Patent Application H09(1997)-266311, disclose lateral semiconductor devices with an alternating conductivity type layer and methods, common to the lateral semiconductor devices and vertical semiconductor devices, for forming the alternating conductivity type layer which employ selective etching technique for digging trenches and epitaxial growth techniques for filling the trenches. In manufacturing the lateral semiconductor device, it is relatively easy to employ selective etching techniques and epitaxial growth techniques to form an alternating conductivity type layer, since thin epitaxial layers are laminated one by one.
However, it is difficult to employ the selective etching technique for digging trenches and using an epitaxial growth technique for filling the trenches in manufacturing the vertical semiconductor devices with alternating conductivity type layer as explained with reference to U.S. Pat. No. 5,216,275. Japanese Unexamined Laid Open Patent Application H09 (1997)-266311 describes the nuclear transformation by a neutron beam and such radioactive beams. However, such nuclear transformation processes require large facilities and cannot be used easily.